This article emphasizes the vital role of pipes in modern infrastructure, tracing their evolution from ancient materials like clay and lead to today’s metal and plastic options. Metal pipes such as cast iron, ductile iron, steel, and copper offer strength and durability but are often heavy, costly, and susceptible to corrosion. Plastic pipes like PVC, HDPE, ABS, and PEX are lighter, more corrosion-resistant, and less expensive, but they generally have lower fire resistance and pressure or temperature tolerance.
The article stresses that proper system design must address factors like water hammer, thermal expansion, and building movement, all of which can damage pipes if not mitigated. Corrosion primarily affects metals, while erosion can degrade all pipe types due to high flow velocity or particulates. Ensuring material compatibility is also critical, as mismatched materials can lead to leaks or premature failure. Ultimately, selecting the right pipe for the specific application is essential for a reliable, long-lasting plumbing system.
Pipe: Choose Wisely for a Successful System
Pipe – the cylindrical structure we so take for granted that contains and conveys fluids between two points. Prior to the advent of pipe, humankind labored to provide water needed for drinking, bathing, irrigation and sustaining livestock. Because of piping, long gone are the days of having to brave the elements when nature calls or, in more civilized circles, to unceremoniously empty the evening’s chamber pot. Now, with a turn of a handle or pull of a lever, we can fill a bath, fill a glass, wash, rinse, or flush without so much as a thought as to how it got there or where it is going. The health and improved quality of life of developed societies owes much to the simple pipe.
PIPE MATERIAL EVOLUTION
Fired or vitrified clay pipe as old as 4,000 years have been found in China. Brick and mortar sewer systems dating as far back as 2500 BC were built by the Persians and Macedonians. Lead pipe, used for its malleability and resistance to corrosion, dates to Roman times and only relatively recently was discontinued in the 20th century due to its toxic effect on humans. Bamboo in tropical areas and hollowed-out logs in Europe and North America have been regionally practical methods used to move water. Presently, metallic piping, consisting of cast iron, ductile iron, steel and copper, and plastic piping consisting of polyvinyl chloride (PVC), high-density polyethylene (HDPE), acrylonitrile butadiene styrene (ABS) and cross-linked polyethylene (PEX), are the types available in the plumbing industry. Each has unique properties that when properly matched to a particular application give the user maximum benefit. On the other hand, incorrectly selected piping material will result in failure of the distribution or collection system. Their respective positive and negative characteristics are as follows.
CAST IRON
Pros: Strong, durable, high load and pressure capacity, corrosion-resistant compared to steel, long lifespan when coated, in corrosive soils its service life may be extended with the application of anodic or cathodic protection, high erosion resistance, fire-resistant, sound-retardant.
Cons: Higher initial cost, heavy, brittle, rough interior surface, susceptible to corrosion if coatings or passive/active corrosion methods are compromised.
DUCTILE IRON
Pros: Greater strength, flexibility, and corrosion-resistance compared to cast iron, resistant to impact, smooth interior surface, in corrosive soils its service life may be extended with the application of anodic or cathodic protection, high temperature rating, sound-retardant.
Cons: High initial cost, heavy, specialized installation requirements, susceptible to corrosion if coatings or passive/active corrosion methods are compromised.
STEEL
Pros: Strong, durable, high load and pressure capacity, wide operating temperature rating, galvanization provides fair resistance to corrosion, sound-retardant.
Cons: Heavy, susceptible to corrosion if uncoated or coatings are compromised, bare metal may affect water quality.
COPPER
Pros: Strong, durable, flexible and lightweight relative to steel and iron, high pressure capacity, wide operating temperature rating, high impact resistance, available in hard-drawn lengths as well as long, soft-drawn coils, chemical- and corrosion-resistant, fire-resistant, will not compromise water quality, 50- to 70-year service life.
Cons: High initial cost, susceptible to highly acidic water resulting in corrosion and metallic taste, high sound and vibration transmission, joining methods require greater skill level.
PVC AND CPVC
Pros: Lightweight, flexible, low material and installation cost, corrosion-resistant, smooth interior surface resulting in lower friction losses, long lifespan, maintains water quality, easily modified.
Cons: Susceptible to ultraviolet (UV) degradation, low fire resistance, low temperature and pressure thresholds compared to metallic pipe, low noise attenuation.
HDPE
Pros: Strong and flexible, high load capacity, high impact resistance, chemical- and corrosion-resistant, high pressure capacity, smooth interior surface resulting in lower friction losses, long service life.
Cons: Susceptible to UV degradation, low fire resistance, high initial cost compared to PVC, specialized equipment required for installation.
ABS
Pros: Lightweight, greater flexibility compared to PVC, low material and installation cost, corrosion- and chemical-resistant, smooth interior surface resulting in lower friction losses, low noise attenuation.
Cons: Susceptible to UV degradation, low fire resistance, high initial cost compared to PVC, not suitable for potable water applications due to the use of Bisphenol A (BPA) in its production.
PEX
Pros: Highly flexible, corrosion-resistant, low material and installation cost, smooth interior surface resulting in lower friction losses.
Cons: Susceptible to UV degradation, low fire resistance, low operating temperature and pressure thresholds compared to metallic pipe.
The stresses imposed on a building’s water distribution and waste collection system must be considered in maximizing piping life and system reliability. Water hammer caused by the rapid deceleration of flowing water will exert forces on joints to the eventual point of failure. It is critical that water hammer arresters are strategically installed throughout the system to absorb this shock.
Thermally, as water is an incompressible fluid and will expand with temperature increases and contract with temperature decreases, this volumetric cycling stresses fittings, ultimately leading to failure. Expansion tanks absorb the volumetric change, effectively removing this source of system stress. Expansion and contraction of the building impose external stresses on both water and waste piping systems. These forces can be substantial and, if piping is not supported in a way that isolates this relative movement between pipe and structure, it is usually the pipe that loses the battle. This is especially the case with cast iron pipe which is inherently brittle and will crack rather than yield to these forces. Having the same susceptibility are pipe repair linings which, when fully cured, are brittle and will crack when sufficient force is applied.
Corrosion and erosion are two more actors affecting piping function. In general, corrosion is limited to metallic piping and whether it is external or internal will compromise the pipe’s wall thickness to the point where its capacity is lower than the operating pressure, resulting in failure. A conducting connection between dissimilar metals will result in localized corrosion. Depending on the pipe material, the chemical aggressiveness of the fluid or salinity of the surrounding air will degrade the pipe. Proper selection of external coatings, isolation of dissimilar metals and, depending on the type of fluid, chemical treatment will mitigate corrosion.
Erosion, however, where the fluid either contains particulates or is flowing at an excessive velocity, or a combination of both, adversely affects both plastic and metallic piping. The progressive thinning of the pipe wall is not readily observable, and the impending failure can be unexpected and catastrophic. Proper selection of pipe size is critical in avoiding this compromising factor.
Finally, compatibility between plumbing materials and the systems to which they are applied is essential to ensure a reliable and efficient plumbing system. Different materials may have different expansion rates, corrosion potentials or joining methods. It is essential to ensure that the selected materials and methods can work together and that appropriate fittings and connectors are available. Incompatibility can lead to leaks, premature failures, and costly repairs. Designers must carefully consider the compatibility of materials to ensure the longevity and functionality of the plumbing system.
